1. Field of the Invention
The present invention relates to a coating film forming method and a coating apparatus each for forming a coating film, for example, a resist film on the front face of a substrate such as a semiconductor wafer or the like.
2. Description of the Related Art
For instance, in a photolithography process of a semiconductor device, a photoresist is applied to a semiconductor wafer, a circuit pattern is transferred to the photoresist using photolithography technology, and the circuit pattern is developed, thereby forming a circuit. In the above process, included is a coating film forming process for applying a resist solution onto the front face of the semiconductor wafer.
In the forming method of a resist film in this case, as shown in FIG. 13, the semiconductor wafer W is mounted on a spin chuck (illustration thereof is omitted) in a processing case and the wafer W is rotated by a drive motor 201. Then, the resist solution is dripped from a resist solution supply nozzle 203 onto the center of the wafer W by a pump 202 such as a bellows pump or the like. Thereby, as shown by many arrows in FIG. 13, the resist solution R is diffused from the center of the wafer W toward the surrounding portion by centrifugal force. Thereafter, the rotation of the wafer W is kept to adjust the thickness of the resist solution R diffused on the wafer W to form a resist film with a uniform film thickness on the wafer W and to blow off an excess resist solution.
As shown by many arrows in FIG. 13, when the resist solution R diffuses from the center of the wafer W toward the surrounding portion by centrifugal force, the circular outline of the outer periphery of the resist solution R spreads while the diameter thereof is increasing by centrifugal force and finally reaches the outer edge of the wafer W keeping its circular form. Thereby, the resist solution R is applied on the entire face of the wafer W in a uniform thickness.
However, when the resist solution R diffuses from the center of the wafer W toward the surrounding portion, as shown in FIG. 14, as for the outline in circular form of the resist solution R, it occurs that the circular form can not be kept and portions thereof which break the circular outline to protrude outward appear in various positions resulting in a waveform outline. This outline in waveform is called "scratchpad", which means that the resist solution R can not spread uniformly on the wafer W. If its degree is serious, "uncoating" of the resist solution occurs and thus there is a danger that a resist film can not be formed on the entire surface of the wafer W uniformly in film thickness.
As the reason why the scratchpad S occurs, it is suspected that centrifugal force acts greatly with the circular outline of the resist solution R spreading outward, thereby increasing the spreading speed of the outline, and as a result, portions breaking the circular outline appear in various positions.
By the way, in recent years, the amount of the resist solution dripped is reduced from the viewpoint of reducing the amount of the resist consumed because of a reduction in fabrication cost and the like. Therefore, a solvent such as a thinner or the like is applied before applying the resist solution to improve wettability of the resist solution, thereby preventing a scratchpad. However, when the amount of the resist is little, a scratchpad is prone to occur, and also when the wettability is poor, a scratchpad is easily to occur.
To avoid "uncoating" by preventing this kind of scratchpad, conventionally it is necessary to apply a resist solution in amount slightly more excessive than originally needed, on the contrary to the demand to reduce as much as possible the consumption of resist in the result.